Studies on morphogenesis and regeneration in cnidarians have a long history, and the importance of cell-ECM (extracellular matrix) interactions for these processes has been well recognized and studied since the middle of the 20th century. Cnidarians have a life cycle with a larva, a polyp, and often a medusa generation. In the medusa, the ECM (mesoglea) is very prominent and essentially shapes the animal. In the larva and the polyp, the ECM is a thin layer. Some of the ECM components known from vertebrates have been identified in cnidarians by immunohistochemistry, electron microscopy, rotary shadowing, biochemistry, and molecular cloning. In vivo and in vitro experiments suggest that the cnidarian ECM plays a role in cell migration and morphogenesis comparable to that known from other developmental systems. In the fresh water polyp Hydra, regeneration of body patterns and migration of nematocytes seems to require the presence of ECM ligands and the corresponding cell receptors. In hydrozoan medusae, DNA replication and the stability of the differentiated state of isolated tissue can be influenced by altering the properties of the ECM substrate. When cultured, most cnidarian cells survive only when attached to ECM substrates, they rarely divide and die within short times. Microsc.